Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is the device itself which includes pulse generator circuitry that generates the pacing stimulation pulses, other circuitry that senses cardiac activity, and a power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing polarity electrode configurations. In unipolar pacing, the pacing stimulation pulses are applied between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. Usually the electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In bipolar pacing, the pacing stimulation pulses are applied between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events represented as P waves on the surface electrocardiogram (ECG) and intrinsic ventricular events represented as R waves on the surface ECG. The pacemaker, however, does not use the surface ECG electrical events but uses the signal as identified inside the heart. This is termed an electrogram. It would be an atrial EGM (AEGM) for the native atrial depolarization and a ventricular EGM (VEGM) for a native ventricular depolarization. By monitoring such AEGM and VEGM, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
The energies of the applied pacing pulses are selected to be above the pacing energy stimulation threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize or contract. If an applied pacing pulse has an energy below the pacing energy stimulation threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the pacing energy stimulation threshold.
However, it is also desirable to employ pacing energies which are not exorbitantly above the stimulation threshold. The reason for this is that pacemakers are implanted devices and rely solely on battery power. Using pacing energies that are too much above the stimulation threshold would result in early depletion of the battery and hence premature device replacement. Prior to autocapture, the capture threshold would be assessed at the periodic follow-up visits with the physician and the output of the pacemaker adjusted (programmed) to a safety margin that was appropriate based on the results of that evaluation. However, capture thresholds may change between scheduled follow-up visits with the physician. A refinement of the technique of periodic capture threshold measurements by the physician was the automatic performance of capture threshold assessment and the automatic adjustment of the output of the pulse generator. Capture threshold may be defined in terms of pulse amplitude, either voltage or current, pulse duration or width, pulse energy, pulse charge or current density. The parameters that can be easily adjusted by the clinician are pulse amplitude and pulse width. With the introduction of autocapture, the implanted pacing system may periodically and automatically assesses the capture threshold and then adjusts the delivered output. It also monitors capture on a beat-by-beat basis such that a rise in capture threshold will be recognized allowing the system to compensate by delivery initially of higher-output back-up or safety pulses and then incrementing the output of the primary pulse until stable capture is again demonstrated. The output amplitude of the pacing stimulus is set slightly above the measured capture threshold minimizing battery drain while the patient is protected by the significantly higher output back-up safety pulse. These evaluations are often referred to as autocapture tests or simply autocapture.
As is well known in the art, the stimulation threshold of a heart chamber can, for various reasons, change over time. Hence, pacemakers that incorporate autocapture are generally able to periodically and automatically perform autocapture tests. In this way, the variations or changes in stimulation threshold can be accommodated.
When a pacing pulse is effective in causing depolarization or contraction of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization or contraction of the heart muscle, it is referred to as “lack of capture” or “loss of capture” of the heart.
When a pacemaker performs an autocapture test, its pulse generator applies a succession of primary pacing pulses to the heart at a basic rate. The output of the primary pulse is progressively reduced. In one known system, there will be a minimum of two consecutive pulses at a given energy before the output associated with the primary pulse is reduced or increased. The output of successive primary pacing pulses is reduced by a known amount and capture is verified following each pulse. If a primary pulse results in loss of capture, a backup or safety pulse is applied to sustain heart activity. If there is loss of capture associated with the primary pulse on two successive cycles, this is interpreted as being subthreshold. At that time, the output associated with the primary pulse is progressively increased in small increments until capture is confirmed on two consecutive primary pulses. This, of course, is but one example. As is known in the art, a single pulse or any number of pulses may be used to establish the capture threshold. The lowest output setting that results in capture on consecutive pulses starting from a low value where there is loss of capture is defined as the capture threshold. A most recent system then automatically adjusts the output with a working margin of an additional 0.25 Volts. In these methods, capture may be verified by detecting the evoked response associated with the output pulse, the T-waves or repolarization waves associated with the electrical depolarization, mechanical heart contraction, changes in cardiac blood volume impedance, or another signature of a contracting chamber.
Fusion and pseudo-fusion beats are commonly encountered in implantable cardiac pacing systems where a native atrial or ventricular intrinsic activation occurs at the same time or within a small window prior to the delivery of a pacing pulse to the same chamber. Accurate assessment of capture threshold in the presence of fusion and pseudo-fusion is a challenge. Fusion beats can confuse capture threshold management algorithms during their normal operation and during the capture threshold search. For example, fusion beats can produce a false-negative result or a false positive result. A false-negative results when a stimulus is determined to be sub-threshold when in fact capture occurred. When a fusion beats occurs, the intrinsic cardiac signal obscures the evoked response, causing the evoked potential to go undetected. In contrast, a pseudo-fusion beat can result in a false-positive classification. Pseudo-fusion occurs when a pacing stimulus occurs coincident with the intrinsic depolarization. With pseudo-fusion beats, however, the pacing pulse does not capture the heart but is detected as capture because the system incorrectly classifies the intrinsic waveform as an evoked response.
If the pacing system paces the heart without evaluating the evoked response, the fusion/pseudo-fusion beats have no effect on either the heart or the pacing system. However, if the pacing system needs to evaluate the evoked responses of the pacing pulse, the fusion/pseudo-fusion distorts the evoked potential such that a false-capture indication can result. When the system has a false-negative capture indication, the system delivers a backup pulse. This can be an undesired behavior, especially in a very sensitive atrium where a backup pulse might induce atrial fibrillation. During a threshold search, false-positive or false-negative capture indications can result in false capture threshold assessment with the resulting capture threshold being either above or below the real capture threshold. For those algorithms which only assess capture threshold periodically instead of on a beat by beat basis, a false capture threshold assessment below the real capture threshold can be catastrophic, since the system may not provide enough stimulus energy to capture the heart. As used hereinafter, the term “fusion beat” will be used to denote both a fusion beat and a pseudo-fusion beat unless a contrary intent is indicated.
Hence, there is a need in the art for an implantable cardiac stimulation device capable of recognizing and responding to fusion beats in such a manner that such fusion beats do not disrupt normal device operation or capture threshold assessment. The present invention addresses these and other issues.